A charge pump is a type of DC-DC converter that uses capacitance instead of inductance to control output voltage. They are limited in output power compared to an inductive converter because of the difference between an inductor and a capacitor. To try to use terms simple as possible, it's the difference between current flowing through a wire coil and charge building up on a set of plates. Current flowing through a wire coil is going to be much less limited in terms of power. The advantage is charge pumps are simple devices and easy to employ.

The charge pump simply boosts voltage so the MOSFET can be driven with a higher voltage than what's available from the battery. This puts the MOSFET in a lower resistance on-state and removes the requirement to use MOSFETs with extra low gate-source threshold. It really frees up the range of MOSFETs that can be used, but also adds more parts. Kind of a 6 of one half dozen of the other situation.

For any current path the limitation is going to be heat. If the current paths on PCB heat up to much that can be a problem as much as MOSFETs overheating themselves. The current paths he's using on the PCB are pretty ample so I imagine the MOSFET would overheat before the PCB traces would. Can't say for sure without torture testing it, but the current tolerances are so high it would not be easy to build a test rig for that, and who wants to burn perfectly good parts anyway.

A charge pump is a type of DC-DC converter that uses capacitance instead of inductance to control output voltage. They are limited in output power compared to an inductive converter because of the difference between an inductor and a capacitor. To try to use terms simple as possible, it's the difference between current flowing through a wire coil and charge building up on a set of plates. Current flowing through a wire coil is going to be much less limited in terms of power. The advantage is charge pumps are simple devices and easy to employ.

The charge pump simply boosts voltage so the MOSFET can be driven with a higher voltage than what's available from the battery. This puts the MOSFET in a lower resistance on-state and removes the requirement to use MOSFETs with extra low gate-source threshold. It really frees up the range of MOSFETs that can be used, but also adds more parts. Kind of a 6 of one half dozen of the other situation.

For any current path the limitation is going to be heat. If the current paths on PCB heat up to much that can be a problem as much as MOSFETs overheating themselves. The current paths he's using on the PCB are pretty ample so I imagine the MOSFET would overheat before the PCB traces would. Can't say for sure without torture testing it, but the current tolerances are so high it would not be easy to build a test rig for that, and who wants to burn perfectly good parts anyway.

Thanks for taking the time to explain that.

I can see how it can be a benefit, or a necessity depending on which MOSFET is used, and I can see how it opens doors to other options which have a higher Rds On.

I guess my real question was in regards to the parts lists between the two projects listed under the OSH Park for David4500. I’m still learning here, so please excuse me if this is an elementary question, but two boards use the same (PSMN1R1-25YLC,115) MOSFET, one with the charge pump and one without. At least that’s what he selection in both the parts list/project carts on mouser. I understand I can use whatever I want within the same package size, but I just want to compare the two boards using the same MOSFET.

Based on the spec sheet chart Figure 7, it looks like the fet is right on the curve? So as the charge drops low, or if there is a great deal of voltage drop across the coil, the fet resistance will shoot up to an unsafe zone?

Am I reading the spec sheet correctly?

If not using a charge pump, you basically want as low as a resistance as possible for drain-source on-state resistance correct? So if one fet is 1.5mOhms and the other is 2.0mOhms at the same voltage, than the lower resistance 1.5mOhms selection would be better?

For any MOSFET used as a switch you want the on-state to be in the flat part of the curve. As the consideration is "gate-source voltage versus on-state resistance" the gate-source threshold and on-state resistance stated in the data sheet determine the shape of the curve. You always want an on-state resistance as low as possible and you always want the applied gate-source voltage to put the transistor in the flat part of the curve.

You may wonder why MOSFETs vary so much in these specifications. There is a reason for it. There are trade-offs with respect to the speed and cost of a MOSFET. Some applications may require a fast switch and the trade-off there comes in on-state resistance. For some applications on-state resistance is not as important. It becomes of great importance when dealing with higher currents. Some applications may require higher voltage tolerance and the trade-off there comes in higher gate-source threshold.

There's also the advancement of tech. Transistors improve over time getting faster with lower on-state resistance. Parts become more available at wider ends of the spectrum such as transistors with very low gate-source threshold. The through-hole parts are pretty much obsolete in terms of new parts which don't come in through-hole packages. Though through-hole parts are handy for free wiring stuff, they are limited in performance.

Ok... on that board, it still should function ok, it's just that a pull down resistor was recommended to be added to the series link mosfet to discharge to gate while in series and idle. Don't think that's the cause of the heat issue.

One end of the resistor is connected to a trace leading to the Bat2- pad. The other end is connected to a trace leading to the series link mosfet gate by why of a via (red highlighted hole on left). Along that same trace, the via on the right connects to the lower middle pad of the slide switch. The vias aren't pads... but maybe you could manage to solder a through hole resistor to one of them and the Bat2- pad.

Hi everyone, this is my first post here in this forum, but have been lurking a long time. Some very interesting stuff discussed here! I have been thinking of drawing up a simple mosfet board for a wooden mod I have, and have had various pcb's made for guitar effects pedals (very low current devices) at a hobby level in the past. So thought I would try a pcb for my mod, but this high current stuff seems to be a totally different ball game!

I have a question regarding the current carrying capacity of copper traces. According to online calculators, if I use a trace from say the drain of a PSMN1R1-25YL and its 200 mil wide and say 100mil long (to get to the atty negative pad) and I'm using 1 oz copper, that trace will only handle 13.4A for a 30 degrees C temp rise. I used this calc (but the several ones I used to cross-check each other were similar)

Yet the resistance for the track is given as 0.2 milliohms. Given such low resistance, how can it only be rated to take 13.4A? And with a 30 degree C (86F) temperature increase? Does the intermittent use factor help us, given that we can't vape continually?

You can see the resistance is negligible, am I missing something? 2 oz copper is considerably more expensive

I have another question regarding the use of the MHP-TA15-9-77 as a resettable fuse. It looks to me that its actually a temperature cut-off device designed to be installed in a lipo battery pack, so it can sense temperature and cut the power if the temp reaches 77 degrees C. Am I missing something here too?Any input appreciated guys.

BTW for those that just can't wrap their head around the pcb layout softwares available (like me), I eventually found a quick and easy one that got me going. You don't need a schematic to make it work, you can just draw the pcb, and easy to learn. Its called Sprint Layout and I use version 5.0, I don't like the later one. Its not free, it was about $50 when I got it years ago, but they have a free trial version that will let you see if you like it or not.

I have another question regarding the use of the MHP-TA15-9-77 as a resettable fuse. It looks to me that its actually a temperature cut-off device designed to be installed in a lipo battery pack, so it can sense temperature and cut the power if the temp reaches 77 degrees C. Am I missing something here too?Any input appreciated guys.

I am a bit new at this too but the spec sheet would lead me to believe the same thing. The vendor website does show a Hold/Trip current values for amperage but its design application for LiPoly packs does make a lot of sense too.

Exceeding the current hold of 15 amps, the temp of the fuse will rise along with the resistance of the fuse. If it reaches the thermal cutoff, the fuses will fully trip and cutoff the flow of current. Removing the over current, the fuse will cool off and return to its normal state.

On the data sheet, check out page 2, under "design concept" which detail how the fuses function.

Using two of those 15 amp fuses in parallel, with a mosfet, and installed in a mod, I've intentionally shorted the 510 with a piece of metal. The fuses functioned exactly as them should. The current flow was stop quite quickly. Once the fault was removed, the fuses were slightly more resistive but settled back to normal once cooling off.

It appears that it functions for our purpose then, maybe the bimetal strip heats up anyway with the high current and breaks the circuit.

From this datasheet, it clearly states "During an abnormal event, the device reacts to the rise in cell temperature causing the bimetal contact to open at the specified temperature". Hence the reason for my question. It looks like a good alternative to polyfuses and one-shot fuses, and they are cheap, I'll get a couple and test them. I might get a MHP-TA15-9-72 to compare to the MHP-TA15-9-77 as well.

Are you using 1 oz copper on your boards David, or 2 oz? Have you ever felt the large copper areas at the drains and sources getting warm during your testing?

Actually I just realized I have some 1 oz copper-clad board here somewhere, maybe I should cut a small piece off, solder a wire on each end and run some current through it, to see if my intended track width and length will handle the intended current.

1oz on Oshpark. I'd have to get them somewhere else for 2oz. I just tin the exposed traces or copper fills and it's worked out.

None of the boards or mosfets have been excessively warm after firing. One with 3 parallel mosfets I fired a 0.2 ohm coil for 4 minutes straight. There was a slight temp increase on the board, that was it. No where near hot or burning. I'll get an infrared temperature gun so I can get some actual temp readings.

Sounds like the 1 oz copper has it all running sweet if it only gets a bit warm after 4 mins firing 0.2 ohms. Very nice.

On second thoughts, you are spreading the power between 3 mosfets, each is only taking 1/3 the power, and there would be more copper for the three compared to just one. Also, I stumbled across a PSMN1Rsomething datasheet suggesting 2 oz copper, but that is for a single mosfet at max power with a 1" square of copper on the pcb to act as a heatsink. I'm gonna have to be careful because I only want to use one mosfet, due to not much room in there. Thanks for your comments, very much appreciated.

David, I have a quick question. On all of your boards with solder points for the vm, How much space saving could be accomplished if it were just combined with atomizer + & - ? I know it probably would be very little if at all but just throwing that out there.

My Mouser order came in (easy to run up a tab over there) yesterday and I couldn't wait to get the iron fired up... Just sharing.

My first board (thanks David4500) for sharing your project.

My only concern is the tinning of the traces for the MOSFETs... not sure if I need to lay on some more solder to make it a bit thicker... any suggestions? (see Picture 2)

I also whipped up a test bed with a 510 connection using some banana jacks... it also doubles as a ohm meter breakout adapter for a DMM. Added a little push button switch to it as well to make REL/Zeroing out the lead resistance easier.

Nice! Looks like you've got plenty of solder on the traces. I think I fired that board for 4 minutes continuous and it barely even rose in temperature.

Good to know... I wanted to test this extensively but I started thinking about each individual MOSFET... is there a way to test each one to make sure none of them were damaged from soldering/heat/or ESD?

I am very excited to get this thing into a box... it will be my FIRST DIY mod.

I was thinking of going with 3 x 18650 and hopefully cram it all into a 1590B, but not sure how I should do that since each fuse on this board was supposed to have its own dedicated connection to the positive on each cell. I was thinking of just making the bridge/parallel connection at the battery sled level and then both fuses would give me a combined 30A hold / 60A trip for the entire bank of batteries.

I might be better off just using 2 x 18650 in a 1590G.

Another thing I noticed was my DMM picking up 0.0407v @ the 510 connection at rest... im guessing there is some voltage leakage / idle drain?

Im ordering your 555 board David as soon as ya say its golden which I may have missed the post, but if its golden ya-- gonna poss. put it in a aa 2s box mod, it will fit perfectly per specs and look awesome..

I don't think so, otherwise you would be pulling near 50mA of current from the battery all the time with a 1 ohm coil. I have one mod with a mosfet in it and it doesn't have any voltage at all when off, its 0.000V across the coil. Your circuit board looks like a clean build too. No idea on how to check each individual mosfet at this stage. If I think of something I'll let you know.

On another point, these mosfet-switched devices have no reverse-polarity protection. If the battery is put in backwards it appears the body diode conducts and fires the atty, with the 0.7V approx diode drop. Is there anything else to be aware of? Is there an easy reverse-polarity protection circuit for a mosfet?

Im ordering your 555 board David as soon as ya say its golden which I may have missed the post, but if its golden ya-- gonna poss. put it in a aa 2s box mod, it will fit perfectly per specs and look awesome..

I had some issues on the first version.

Mainly the pfet pads for gate and source were reverse of what they should have been. Diodes were in the wrong direction causing the duty cycle to increase when turning counterclockwise instead of clockwise.

I tested the MHP-TA15-9-77 today, it looks really good, much to my surprise.It will hold 15A no worries.At 22A goes open in around 7 secs.28A 5 secs.43A maybe 1 sec.47A maybe half a second if that.It resets pretty quick and the resistance at 10A flow was measured around 2.25 milliohms, (22.5mV AC on Fluke187), at 5.5A it was about 1.9 milliohms (10.5mV AC).

The testing was done with a high current transformer from a circuit breaker test set, fed with a variac, so AC, but I think the results should be OK and in the ballpark.